The objective of this study is to analyse, from a legal point of view, the influence of the transposition of Marine Spatial Planning Directive into both Spanish and Portuguese domestic laws on the development of marine renewable energies in both countries. This article concludes that the Portuguese legal system is more favourable for the development of marine renewable energies than the Spanish legal regime, since the former establishes a more flexible planning system, sets criteria for the prioritisation of marine uses, incorporates trade-off mechanisms, introduces an electronic single-window system and regulates a pilot zone. These measures can help streamline licensing processes, avoid and resolve conflicts with other sea users, and adapt planning instruments to the rapid development of new marine renewable technologies. However, both legal regimes lack specific legal mechanisms aimed at offering effective protection of the marine environment against negative effects arising from the installation of such devices. Similarly, there is a lack of coordination between maritime spatial planning instruments and land planning instruments, and between the Central Government and the autonomous regions. This may hinder the installation of marine renewable energies. This study has implications in relation to the EU integrated marine policy aimed at achieving a balance between blue growth and the conservation of the marine environment, as well as an inter-administrative coordination improvement in decision-making.